1. Preheat the oven to 375°F. In a large bowl, combine the ham, pork, bread crumbs, onion, egg, milk, salt, and pepper.
2. Butter eight 2-inch muffin pan cups and pack the mixture into the cups, mounding it slightly in the centers but not letting it touch the top edges of the cups. Bake for 20 minutes.
3. Meanwhile, in a small saucepan, combine the sugar, vinegar, and mustard. Bring to a boil over moderately high heat, then cook stirring, for 1 minute, or until the sugar dissolves. Taste for seasoning, adding another tablespoon of vinegar and teaspoon of mustard if desired.
4. Pour the sauce evenly onto the ham mixture in the cups and bake an additional 15 minutes. Remove the loaves from the muffin pan, and serve hot with prepared horseradish or mustard on the side.